Relative scales share the same notes, for example, C major and A Minor. Even though they start at different places, they contain all the same notes and chords.

C Major Scale

Let's also look at the chords built on top of the scale degrees.

A Minor Scale

You can find more examples of relative scales all through out music. The major scale actually has 5 other relative modes. A 'mode' is just another way to say 'relative scale'. Another example would be the Dorian mode, or the Phrygian mode.

D Dorian

E Phrygian

Each of these modes can be explored in their own ways. Dorian and Phrygian are two of the most popular modes to center compositions around. They share the same basic structure of whole steps and half steps as the major scale, and from the right perspectives share the same notes. I talk about this in my modal theory post if you'd like to learn more.